Orographic precipitation

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Orographic precipitation is commonly regarded as a distinct type, but this requires careful qualification. Mountains are not especially efficient in causing moisture to be removed from airstreams crossing them, yet because precipitation falls repeatedly in more or less the same locations, the cumulative totals are large. An orographic barrier may produce several effects, depending on its alignment and size. They include: (1) forced ascent on a smooth mountain slope, producing adiabatic cooling, condensation and precipitation; (2) triggering of conditional or convective instability by blocking of the airflow and upstream lifting; (3) triggering of convection by diurnal heating of slopes and up-slope winds; (4) precipitation from low-level cloud over the mountains through 'seeding' of ice crystals or droplets from an upper-level feeder cloud (Figure 5.14); and (5) increased frontal precipitation by retarding the movement of cyclonic systems and fronts. West coast mountains with onshore flow, such as the Western Ghats, India, during the southwest summer monsoon; the west coasts of Canada, Washington and Oregon; or coastal Norway, in winter months, supposedly illustrate smooth forced ascent, yet many other processes seem to be involved. The limited width of some coastal ranges, with average wind speeds, generally allows insufficient time for the basic mechanisms of precipitation growth to operate (see Figure 5.9). In view of the complexity of

Seeder And Feeder Clouds

Figure 5.14 Schematic diagram of T. Bergeron's 'seeder-feeder' cloud model of orographic precipitation over hills.

Note: This process may also operate in deep nimbostratus layers. Source: After Browning and Hill (1981). Reprinted from Weather, by permission of the Royal Meteorological Society. Crown copyright ©.

Figure 5.14 Schematic diagram of T. Bergeron's 'seeder-feeder' cloud model of orographic precipitation over hills.

Note: This process may also operate in deep nimbostratus layers. Source: After Browning and Hill (1981). Reprinted from Weather, by permission of the Royal Meteorological Society. Crown copyright ©.

Precipitation Hills

Figure 5.15 Mean annual precipitation (1951 to 1980) along a transect of the South Island of New Zealand, shown as the solid line in the inset map. On the latter, the dashed line indicates the position of the Godzone Wetzone and the figures give the precipitation peaks (cm) at three locations along the Godzone Wetzone.

Sources: Chinn (1979) and Henderson (1993), by permission of the New Zealand Alpine Club Inc and T.J. Chinn.

Figure 5.15 Mean annual precipitation (1951 to 1980) along a transect of the South Island of New Zealand, shown as the solid line in the inset map. On the latter, the dashed line indicates the position of the Godzone Wetzone and the figures give the precipitation peaks (cm) at three locations along the Godzone Wetzone.

Sources: Chinn (1979) and Henderson (1993), by permission of the New Zealand Alpine Club Inc and T.J. Chinn.

processes involved, Tor Bergeron proposed using the term 'orogenic', rather than orographic, precipitation (i.e. an origin related to various orographically produced effects). An extreme example of orographic precipitation is found on the western side of the Southern Alps of New Zealand, where mean annual rainfall totals exceed 10 metres! (Figure 5.15).

In mid-latitude areas where precipitation is predominantly of cyclonic origin, orographic effects tend to increase both the frequency and intensity of winter precipitation, whereas during summer and in continental climates with a higher condensation level the main effect of relief is the occasional triggering of intense thunderstorm-type precipitation. The orographic influ ence occurs only in the proximity of high ground in the case of a stable atmosphere. Radar studies show that the main effect in this case is one of redistribution, whereas in the case of an unstable atmosphere precipitation appears to be increased, or at least redistributed on a larger scale, since the orographic effects may extend well downwind due to the activation of mesoscale rain bands (see Figure 9.13).

In tropical highland areas there is a clearer distinction between orographic and convective contributions to total rainfall than in the mid-latitude cyclonic belt. Figure 5.16 shows that in the mountains of Costa Rica the temporal character of convective and orographic rainfalls and their seasonal occurrences are quite

Figure S.I6 Orographic and con-vective rainfall in the Cachi region of Costa Rica for the period 1977 to 1980. (A) The Cachi region, elevation 500 to 3,000 m. (B) Typical accumulated rainfall distributions for individual convective (duration 1 to 6 hours, high intensity) and orographic (1 to 5 days, lower intensity except during convective bursts) rainstorms. (C) Monthly rainfall divided into percentages of convective and orographic, plus days with rain, for Cachi (1018 m).

Source: From Chacon and Fernandez (1985), by permission of the Royal Meteorological Society.

Source: From Chacon and Fernandez (1985), by permission of the Royal Meteorological Society.

Orographic Weather

Figure S.I7 Classic view of the cycle of a local thunderstorm. The arrows indicate the direction and speed of air currents. (A) The developing stage of the initial updraft. (B) The mature stage with updrafts, downdrafts and heavy rainfall. (C) The dissipating stage, dominated by cool downdrafts.

Source: After Byers and Braham; adapted from Petterssen (1969).

distinguishable. Convective rain occurs mainly in the May to November period, when 60 per cent of the rain falls in the afternoons between 12:00 and 18:00 hours; orographic rain predominates between December and April, with a secondary maximum in June and July coinciding with an intensification of the trades.

Even low hills may have an orographic effect. Research in Sweden shows that wooded hills, rising only 30 to 50 m above the surrounding lowlands, increase precipitation amounts locally by 50 to 80 per cent during cyclonic spells. Until Doppler radar studies of the motion of falling raindrops became feasible, the processes responsible for such effects were unknown. A principal cause is the 'seeder-feeder' ('releaser-spender') cloud mechanism, proposed by Tor Bergeron and illustrated in Figure 5.14. In moist, stable airflow, shallow cap clouds form over hilltops. Precipitation falling from an upper layer of altostratus (the seeder cloud) grows rapidly by the wash-out of droplets in the lower (feeder) cloud. The seeding cloud may release ice crystals, which subsequently melt. Precipitation from the upper cloud layer alone would not give significant amounts at the ground, as the droplets would have insufficient time to grow in the airflow, which may traverse the hills in 15 to 30 minutes. Most of the precipitation intensification happens in the lowest kilometre layer of moist, fast-moving airflows.

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  • Ursula
    What is orographic precipitation?
    8 years ago
  • belinda
    What is orographic intensification?
    5 years ago
  • Primula
    Where is orographic precipitation mostly found?
    3 years ago

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